1. Field of the Invention
The invention relates generally to temperature compensation in electrochemical sensors. More specifically, the invention relates to automated compensation for temperature-related error in an intravenous amperometric sensor.
2. Description of Related Art
There are many applications for electrochemical sensors. One class of electrochemical sensors, known as amperometric biosensors, has many potential applications in the health care industry. For example, recent advances in the art of glucose monitoring are considering systems that continuously monitor blood glucose levels using an intravenous amperometric glucose sensor installed in a large blood vessel. The intravenous sensor continually outputs an electrical signal representing blood glucose level to a computer system that displays the level in real time for the attending physician. This allows the physician to take corrective action in case the blood glucose level becomes too high or too low. In an ICU or other emergency situation, the ability to immediately monitor and control blood glucose levels, particularly for diabetic patients, can mean the difference between life and death.
The accuracy of the output of the biosensor is therefore very important. Among many variables that can affect sensor accuracy is temperature. An amperometric sensor is typically calibrated at normal body temperature. When the sensor is immersed in the patient's bloodstream, it produces an electrical current in response to chemical reactions that occur on the surface of the sensor. Changes in body temperature are likely to affect the reaction rate, causing a loss of accuracy as the sensor outputs more or less current than it would at normal body temperature. The infusion of fluids through a catheter in the immediate vicinity of the sensor can also cause temperature-related errors if the infused fluids have a temperature different than the body temperature. The sensors can also be affected by exposure to room temperatures prior to insertion into the body. Depending on the location of the biosensor and the configuration of the device in which the biosensor is located, temperature changes may cause the current produced by the biosensor to change for the same glucose concentration, thereby invalidating the calibration curves. This may cause the accuracy of these biosensors to be unacceptable for clinical use and perhaps unreliable for guiding therapy.
A prior solution addressing the temperature-dependency problem of amperometric sensors includes withdrawing a sample of blood and measuring the glucose level in an isolated static environment with constant temperature. This solution, however, is not practical for use in an ICU, where time is of the essence. Another solution involves withdrawing a sample of blood across a biosensor and recirculating the blood back to the patient. This solution adds considerably to the complexity of the monitoring system and is difficult to implement in practice, due to the limited number of patient access sites, which are usually reserved for transfusions of blood and medicines. These solutions do not compensate for the temperature changes but rather seek to avoid the possibility of temperature changes.
Another solution involves using a separate temperature sensing element, such as a thermistor or a silver trace or other device, having a resistance that changes with temperature. However, the separate sensing element adds complexity to the monitoring system, takes up additional space within the catheter, and adds to the risk of infection, among other disadvantages.
With an increasing trend toward using amperometric sensors in health care and other industries, and especially in view of ongoing demand for improvements in glucose monitoring, a need exists for a more practical solution for temperature compensation in biosensor electrodes to provide reliable measurements despite a change in surrounding temperature.